[Support] Expose flattenWindowsCommandLine.

This function was internal to Program.inc, but I've needed this
on several occasions when I've had to use CreateProcess without
llvm's sys::Execute functions.  In doing so, I noticed that the
function was written using unsafe C-string access and was pretty
hard to understand / make sense of, so I've also re-written the
functions to use more modern LLVM constructs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334353 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Program.cpp

@@ -63,6 +63,15 @@ ProcessInfo sys::ExecuteNoWait(StringRef Program, const char **Args,
   return PI;
 }
 
+bool sys::commandLineFitsWithinSystemLimits(StringRef Program,
+                                            ArrayRef<const char *> Args) {
+  SmallVector<StringRef, 8> StringRefArgs;
+  StringRefArgs.reserve(Args.size());
+  for (const char *A : Args)
+    StringRefArgs.emplace_back(A);
+  return commandLineFitsWithinSystemLimits(Program, StringRefArgs);
+}
+
 // Include the platform-specific parts of this class.
 #ifdef LLVM_ON_UNIX
 #include "Unix/Program.inc"